Synthetic auxinic herbicides were developed around sixty years ago and contributed greatly to changes in agricultural and horticultural practices. These synthetic molecules, copied from the endogenous, or natural, auxins found in plants, are able to control or eliminate several weeds, primarily large-leaved dicotyledonous weeds, generally without overly affecting monocotyledonous plants, primarily grasses. Today, this type of herbicide is used in a wide variety of crops, and even on unfarmed land. Currently, synthetic auxinic herbicides play a major role in managing weeds, whether used alone or with other herbicides or products. Their low production cost and the wide selectivity spectrum developed over the years explain their great success. Indeed, synthetic auxinic herbicides are divided into four classes based on their chemical structure. These four classes, distributed into various sub-classes incorporating several different products, allow a selective control of various species of dicotyledonous weeds found in the agriculture sector as well as the nonprofessional horticulture sector, such as turf management.
The action mode of synthetic auxinic herbicides for sensitive plant species is characterized by an uncontrolled growth often described as an “auxin overdose.” The applied dosage causes a variety of metabolic anomalies or disruptions, evolving over time until in most cases, the plant dies. Despite their broad usage spectrum over the years, the exact action mechanism is not fully known, given the cascade of reactions that affect different biochemical pathways in the plant.
2,4-D, or 2,4-dichlorophenoxyacetic acid, is one of the most commonly used synthetic auxinic herbicides. Indeed, it is found in the formulation of several hundred commercially available products, in particular those sold for weed control in turf. Over the last two decades, several studies have shown environmental and human health risks resulting from the use of certain synthetic auxinic herbicides, in particular 2,4-D. Faced with this situation, several countries and territories have decided to ban the use of 2,4-D for lawn maintenance, for example, given the closeness of homes and the purely aesthetic considerations of this practice. In the regions where synthetic auxinic herbicides are prohibited, or in regions without regulations but where users are aware of the issue, various alternative solutions have been developed or studied. However, it appears that to date, these solutions have found only limited success.
The development of a new low-risk selective herbicide for agriculture and horticulture is faced with several challenges, often of competing natures, which makes the approach much more complex. On the one hand, the active ingredient used must be stable enough to withstand various storage and application conditions, but easy enough to break down after application to prevent it from building up in the environment. On the other hand, the active ingredient must lead to quick and obvious symptoms able to comfort the user, in particular for the amateur horticulture sector, but without endangering the desired long-term effect. In other words, the above ground part of the plant must be clearly affected, but without harming the progression of the molecule or product toward the roots in order to ensure permanent elimination of the targeted plant. Lastly, the active ingredient must irreversibly affect the metabolism of the harmful plants or weeds without altering, or excessively disrupting, the growth of the farmed, beneficial or desirable plants. Indeed, the efficacy of a selective herbicide is based on two main parameters, namely the control of the weed to be eliminated and the selectivity of the effect with respect to the desirable plants. These two parameters are influenced by various factors related to the plant itself, the outside environment and the nature of the active ingredient.
In connection with the plant, these factors in particular include:                how the plant or leaves are carried (spread versus upright),        the width and shape of the leaves,        the position or exposure of the meristem,        the thickness of the cuticle of the leaves,        the density of the trichomes on the leaves,        the age of the plant,        etc.        
In terms of the prevailing environment, these factors in particular include:                the climate conditions,        the period of the growing season,        the time of day,        the type of application (targeted versus spreading),        etc.        
In terms of the nature of the active ingredient, it is in particular necessary to consider:                the movement or transport pathways of the substance in the plant,        the interaction with the protein receptors of the plant,        the mechanisms involved in the action of the substance,        the type of physiological disruptions caused,        etc.        
There is no solution with a low environmental risk that has been made commercially available in recent years that meets all of the desired criteria. Several alternatives are described here as examples, for the lawn care sector. Acetic acid may kill several species of weeds, but the product is not selective enough. The concentration of acetic acid used to eliminate dicotyledonous plants with wide and spread leaves, such as dandelion, also affects grasses with narrow, upright leaves such as turf. This type of product can therefore be sold only as a nonselective herbicide. The use of an aqueous solution containing from 8 to 20% sodium chloride is also recommended (U.S. Pat. No. 6,372,690 B1), but it appears that this type of product may be difficult to use for spreading-type applications. Furthermore, even in a targeted-type application mode, the product may cause the turf to yellow and have a significant regrowth rate under certain climate conditions. Other products are based on the use of metals such as iron, in chelate form in solution (U.S. Pat. No. 6,972,273 B2). To go beyond the surface or visible necrosis effect alone, several applications are systematically needed.
Such a situation is incompatible with targeted applications. To overcome this difficulty, another invention combines a low concentration of synthetic auxinic herbicide, of the 2,4-D type, with a metal in chelate form (U.S. Pat No. 8,076,267 B2). Although this strategy represents a certain environmental gain, it does not allow access to the growing number of territories where synthetic auxinic herbicides are banned. Furthermore, it should be noted that the chelating agents most commonly used in this type of commercial product, i.e., ethylenediaminetetraacetic acid (EDTA) and hydroxyethylenediaminetriacetic acid (HEDTA), are also implicated in some studies, given their potential buildup in the soil. Work has also been done to use an overdose of indole acetic acid (IAA), a natural plant auxin, to develop a selective herbicide. This endogenous auxin has demonstrated a selective herbicide-type effect, but it is short lasting. The molecule is metabolized quickly by the plant and/or its bond with the protein receptors causes an excessively weak action. Other works have proposed the use of herbicides with a base of synthetic or natural auxins, like IAA, for genetically modified plants after introducing one or several specific genes blocking ethylene synthesis (U.S. Pat. No. 5,670,454). This type of selective herbicide is of limited usefulness, since it is applicable only to certain genetically modified plants. Lastly, it should be noted that in the territories where synthetic auxinic herbicides are prohibited, various decoctions or products of plant origin have a limited effect, such as beet juice and corn gluten. These partial solutions have an effect, following a buildup on the surface or in the soil, on the germination of weed seeds in a pre-established turf.
Work has also been done with another endogenous or natural auxin, having a higher “auxinic” physiological action than IAA, that is found only in a few plant families, namely 4-chloroindole-3-acetic acid (4Cl-IAA). This molecule has for example been found in peas at a given stage of the plant's development. However, Engvild (“Herbicidal activity of 4-indole acetic acid and other auxins on pea, barley and mustard,” Physiol. Plant. 96:333-337, 1996) observed in the laboratory that 4Cl-IAA, dissolved in an aqueous solution containing 10% ethanol, had an impact about 4 times lower on the growth of dicotyledonous plants (peas and mustard) compared to 2,4-D. Conversely, the sensitivity of the studied monocotyledonous plant (barley) to 4Cl-IAA was three times higher, still compared with 2,4-D. Indeed, a quantity of 0.17 g/m2 of 4Cl-IAA was sufficient to affect the growth of the monocotyledonous plant (barley) by 50%, versus 0.5 g/m2 with 2,4-D. Similar trends were observed in terms of mortality, expressed in lethal dose for 50% of the plants (LD50). Furthermore, the study showed that from one dicotyledonous plant to the other (peas versus mustard), the actual quantities varied greatly, namely by a factor 5. All of these results brought to light a certain capacity to affect vegetation, but appear incompatible with the development of a selective herbicide able to replace 2,4-D. Lastly, a Chinese patent application (CN 103621505 A) was filed on halogenation, in different positions of the carbon cycle, of an IAA molecule for use as an herbicide having concentrations varying from 5 to 90% on a weight basis or 50 to 900 g/L. Considering the low aqueous solubility of this type of molecule (pKa of about 4), an herbicide containing the concentrations recommended in the patent application involves the use of a high organic solvent content that may affect more sensitive grasses and reduce the selectivity of the product accordingly. Furthermore, it should be stressed that a product involving such high concentrations may cause spreading difficulties depending on the required doses.
Therefore, as one can see, there is still a need for a low-risk, selective and effective herbicide able to be used for targeted and/or spreading applications in the professional or amateur horticulture sector and in agriculture.